Electron Configuration Practice with Answer Key

Electron Configuration Practice: The Only Drill You Need

You need to write electron configurations. You need to verify if you're right. This guide gives you both — 40 practice problems with a complete answer key.

No fluff. No inspirational quotes. Just electrons filling orbitals the way they actually do.

What You Actually Need to Know First

Before you start the problems, make sure these rules are locked in your brain:

• Aufbau Principle: Electrons fill lowest energy orbitals first. The diagram below shows the order.
• Pauli Exclusion Principle: Each orbital holds maximum 2 electrons with opposite spins.
• Hund's Rule: Electrons spread out in degenerate orbitals before pairing up.

The Orbital Filling Order (Memorize This)

Most students draw this as a diagonal diagram. Here's the sequence:

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p

Or easier — fill this pattern diagonally:

1s
2s 2p
3s 3p 4s 3d
4p 5s 4d 5p 6s 4f
5d 6p 7s 5f 6d 7p

Electron Capacity Per Subshell

• s subshell: 2 electrons
• p subshell: 6 electrons
• d subshell: 10 electrons
• f subshell: 14 electrons

Practice Problems

Level 1: Basic Configurations (Problems 1–10)

Write the full electron configuration for each element:

1. Hydrogen (H)
2. Helium (He)
3. Lithium (Li)
4. Carbon (C)
5. Nitrogen (N)
6. Oxygen (O)
7. Neon (Ne)
8. Sodium (Na)
9. Chlorine (Cl)
10. Argon (Ar)

Level 2: Transition Metals and Beyond (Problems 11–25)

11. Potassium (K)
12. Calcium (Ca)
13. Iron (Fe)
14. Copper (Cu)
15. Zinc (Zn)
16. Bromine (Br)
17. Krypton (Kr)
18. Silver (Ag)
19. Gold (Au)
20. Lead (Pb)
21. Mercury (Hg)
22. Platinum (Pt)
23. Nickel (Ni)
24. Manganese (Mn)
25. Chromium (Cr)

Level 3: Ions and Exceptions (Problems 26–40)

26. Fe²⁺ (Iron ion with 2+ charge)
27. Fe³⁺ (Iron ion with 3+ charge)
28. Cu⁺ (Copper ion with 1+ charge)
29. Cu²⁺ (Copper ion with 2+ charge)
30. O²⁻ (Oxide ion)
31. Na⁺ (Sodium ion)
32. Cl⁻ (Chloride ion)
33. S²⁻ (Sulfide ion)
34. Ti²⁺ (Titanium ion)
35. Ti⁴⁺ (Titanium ion)
36. Co²⁺ (Cobalt ion)
37. Co³⁺ (Cobalt ion)
38. Zn²⁺ (Zinc ion)
39. Sn²⁺ (Tin ion)
40. Sn⁴⁺ (Tin ion)

Answer Key

Level 1 Answers

Level 2 Answers

Level 3 Answers (Ions and Exceptions)

Where Students Actually Screw Up

1. Forgetting the 4s Fills Before 3d

Calcium is 4s², not 3d². The 3d orbitals don't start filling until after you've done 4s². This trips up almost everyone at first.

2. Thinking Ions Lose Electrons From the Highest Number

Wrong. Positive ions always lose from the outermost s orbital first, then the d. For Fe²⁺, you remove both 4s electrons before touching 3d.

3. Missing the Exceptions

Chromium should be 4s² 3d⁴, but it's actually 4s¹ 3d⁵. Copper should be 4s² 3d⁹, but it's 4s¹ 3d¹⁰. Half-filled and fully-filled subshells are more stable. Memorize these two exceptions and you'll be fine.

4. Writing Noble Gas Notation Incorrectly

[Ar] means everything through argon. [Kr] means everything through krypton. You can only use a noble gas notation if it actually matches the beginning of your configuration. You can't use [Ne] for iron — that's wrong.

Quick Reference: Common Noble Gas Cores

How to Practice Effectively

Don't just read these. Write them out by hand.

1. Pick 10 elements from the list above. Close the answer key.
2. Write the full configuration from memory. Include every orbital.
3. Check your answers. Find every mistake and understand why.
4. Repeat daily until you can do any configuration in under 2 minutes.

That's it. There's no secret method. Repetition fixes this.